1. Field of the Invention
The present invention relates in general to the technique of passivation, and in particular to a new and useful process and solution for passivating metal surfaces in a nuclear steam generator.
2. Description of the Related Art
Passivation can be defined in simple terms as the process which causes the loss of chemical reactivity in certain chemical environments. A detailed discussion of passivation can be found in M. G. Fontana and Greene, Corrosion Engineering, 2nd Edition, New York, N.Y.: McGraw-Hill, 1978, pp 319-324. Several important points are provided in this reference. First, it is evident that passivation is a state where corrosion of a metal in the environment under question is very low. Secondly, the passivation is relatively unstable and is specific to the environment. It is therefore important to understand the purpose of the passivating solution applied after chemical cleaning.
After being chemically cleaned with acids or other solvents designed to remove metal oxides, metal surfaces, especially ferrous based steels, are usually in a highly active state in which they are subject to rapid oxidation in the presence of air. This is often referred to as "flash rusting" or "after rust." The normal purpose of passivation after chemical cleaning is to prevent this flash rusting. Passivation solutions are generally directed to the ferrous based steel materials. The passivation potentials of nickel-based alloys such as Inconel 600, Monel 400, Incoloy 800, or Inconel 690, are much lower than that of iron. These high nickel alloys are essentially self-passivating, spontaneously forming a protective oxide film when exposed to air. The chemical cleaning passivation solution is aimed at preventing flash rusting on steam generator surfaces after chemical cleaning. This becomes especially important in the case where additional maintenance work is required which will result in the cleaned surfaces being exposed to the air. Once the cleaned units are put back into operation, the materials experience a different environment. The chemical cleaning passivation layer is no longer important. The water chemistry employed during operation is designed to promote passivation of the metal surfaces under the operating conditions of the steam generating equipment.
The EPRI/SGOG process utilizes a high temperature (93.degree. C./200.degree. F.) hydrazine solution as the final passivation step after chemical cleaning of the nuclear steam generators. The EPRI/SGOG passivation solution contains approximately 200 ppm of hydrazine with the pH adjusted to about 10.2 with ammonia. This solution has been demonstrated to provide the appropriate passive layer after chemical cleaning (R. D. Martin and W. P. Banks, "Electrochemical Investigation of Passivating Systems," Proceedings of the 35th IWC, Pittsburgh, Pa., 1974, pp 169-179 and R. D. Martin and W. P. Banks, "Electrochemical Investigation of Passivating Mild Steel Surfaces," Materials Performance, Vol 14, Nov. 9, 1975, pp 33-37). Note however, that if a small amount of copper is left in the steam generators, it will tend to plate on the steam generator surfaces during this passivation step.
U.S. Pat. No. 4,654,200 describes a process specific to the leaching of radium from uranium mill tailings through chelation with EDTA under strong reducing conditions. The present invention is not applicable to the uranium industry and is applied under oxidizing conditions (hydrogen peroxide), not the strong reducing conditions specified in this patent. The patent does not deal with passivation of steel surfaces.
U.S. Pat. No. 3,506,576 describes a process for cleaning a ferrous based metal surface using an aqueous alkaline solution of an alkylene polyamine polyacetic acid chelating agent with a water soluble sulfide.
U.S. Pat. No. 4,632,705 describes a process for removal of ferrous or copper-type deposits from the secondary side of nuclear steam generators. The process utilizes primary heat and boiling to transport and concentrate the cleaning agent into the restricted areas of the generator. The present invention is meant to establish a protective oxide layer on all carbon steel surfaces after a chemical cleaning. Some removal of copper-type deposit does occur as a secondary effect. The prior art process is specifically designed to remove ferrous and copper-type deposits within restricted (creviced) regions. The inventive process heats the cleaning agent using external heating systems. The prior art process heats the cleaning agent using primary loop heat. The inventive process heats the cleaning agent using primary loop heat only when needed, to establish the required application temperature of 37.degree. to 49.degree. C. The prior art heats the cleaning agent to induce boiling. The new process also periodically vents the steam generator to release generated gases. The prior art process reduces pressure to induce boiling and has specific pressure requirements (above and below atmospheric pressure). The invention is applied at atmospheric pressure.
U.S. Pat. No. 5,225,087 describes a process for recovery of EDTA from chemical cleaning and decontamination solutions. The EDTA is precipitated by reducing the pH to less than 2.0. The inventive process uses EDTA to establish a protective oxide layer following removal of ferrous deposits during chemical cleaning and does not suggest a process for recovery of the passivation solution to facilitate its disposal or reuse. The prior art is for recovery of the cleaning solution, once the chemical cleaning is complete, to facilitate its disposal or reuse. The inventive process is also applied in the steam generator, but the prior process is applied in tanks external to the steam generator.
Dowell Schlumberger (DS) "Passivation Of Steel In Ammonium EDTA Solution", presented Mar. 25-29, 1985, at Corrosion 85, describes a process for passivating steel after performance of a chemical cleaning. The invention utilizes hydrogen peroxide as an oxidant for the process. The DS process intentionally adds 800 ppm to 4100 ppm iron to achieve the desired concentration of ferric ions. The new process is applied in a temperature range of 37.degree. C. to 49.degree. C. The DS process is applied in a temperature range of 54.degree. C. to 77.degree. C. The inventive process does not require continuous flow of the passivating solution across the steel surface. The DS process specifies a minimum continuous flow rate across the steel surface to ensure effective passivation.